Fuse structure having arc-quenching sleeve



D 13, 1966 P. c. HITCHCOCK FUSE STRUCTURE HAVING ARCQUENGH1NG SLEEVEFiled March 4, l964 2 Sheets-Sheet l Dec. 13, 1966 P. c. HlTcHcocK3,291,939

FUSE STRUCTURE HAVING ARC-QUENCHING SLEEVE Filed March 4, 1964 2Sheets-Sheet 2 INVENTOR:

Paul C. Hitchcock By AMY.

United States Patent O 3,291,939 FUSE STRUCTURE HAVING ARC-QUENCHINGSLEEVE Paul C. Hitchcock, Plum Island, Mass., assigner to TheChase-Shawmut Company, Newburyport, Mass., a corporation ofMassachusetts Filed Mar. 4, 1964, Ser. No. 349,453 7 Claims. (Cl. 20G-120) This invention has reference to electric fuses.

It is one object of this invention to provide electric fuses havingnovel and improved arc-quenching means.

It is another object of this invention to provide electric fuses havingpoint-heat-source type fusible wires with novel and effectivearc-elongating means resulting in a degree of arc elongation by farexceeding the available extremely short backburn length of such wires.

It is another object of this invention to provide electric fuses whereina substantial degree of arc elongation is achieved by means ofarc-runners and gas blast means for propelling the terminals of electricarcs along the arcrunners.

Another object of this invention is to provide an improved version ofthe kind of fuses disclosed and claimed in United States Patent2,856,488 to Frederick I. Kozacka, Oct. 14, 1958, Current-Limiting Fusesfor Small Current Intensities, assigned to the same assignee as thepresent invention.

Another object of this invention is to provide electric fuses similar tothose disclosed and claimed in United States Patent 2,895,031 toFrederick I Kozacka,( July 14, 1959, Fusible Protective Devices,assigned to the same assignee as the present invention, but havingperformance characteristics which are even better than the performancecharacteristics of electric fuses manufactured in accordance with theteachings of the aforementioned patent.

Still another object of this invention is to provide fuses performingsimilar to the fuses disclosed and claimed in United States Patent2,895,031 but being less expensive to manufacture.

For a better understanding of the present invention reference may be hadto the accompanyin-g drawings in which:

FIG. l is a longitudinal section of a portion of a prior art structureforming the point of departure of the present invention;

FIG. 2 is a longitudinal section similar to that of FIG. 1 showing aportion of a structure embodying the present invention;

FIG. 3 is a top-plan view of the structure of FIG. 2;

FIG. 4 is a longitudinal section of the structure of FIG. 2 in theprocess of performing a very severe interrupting duty;

FIG. 5 is a section on a larger scale taken along 5 5 of FIG. 3; and

FIG. 6 is a photornacrograph of the structure of FIGS. 2 to 5,inclusive, upon the performance of a severe interrupting duty close tothe limit of the interrupting capacity thereof.

Referring now to the drawings, and more particularly to FIG. 1 thereof,reference character S has been applied to generally indicate a length ofcoppergclad organi-c insulating material of the kind widely applied inmaking socalled printed circuits. Strip S is a laminate including theinner layer 1 of organic insulating material which is sandwiched betweenthe two outer layers 2 of metal, preferably copper. Strip S is providedwith a narrow circular bore or passage 1a extending transversely acrosslayer 1 and layers 2. A wire 3, preferably of silver, is threadedthrough bore or passage 1a. The outer ends 0f wire 3 are conductivelyconnected to layers 2. Reference numeral 4 has been applied to indicatesolder joints 3,291,939 Patented Dec. 13, 1966 ICC conductivelyconnecting the ends of wire 3 outside of bore or passage 1a to copperlayers 2. Transverse grooves 2a extend all theYway across copper layers2, thus interrupting the current path formed by upper copper layer 2 aswell as interrupting the current path formed by the lower copper layer2. The structure of FIG. 1 thus establishes the following current path:left portion of upper copper layer 2, upper solder joint 4, portion ofwire 3 in bore or passage la, lower solder joint 4, right portion oflower copper layer 2.

FIG. 1 has been drawn on a very large scale. Actually thecurrent-carrying length of wire 3 is sufficiently short to fairlyapproximate a point-heat-source. In other words, the current-carryinglength of wire 3 hardly exceeds the thickness of a sheet of copper-cladlaminate of the kind used in printed circuitry. Because of the shortnessof the current-carrying length of wire 3 and because of the smallness`of the cross-section thereof, the fz-dt Values required for fusing andvaporizing the current-carrying length of wire 3 are very small. Thus anelectric conductor including points of lreduced cross-section formed byone or more serially connected portions of wire inside of `one or morepassages formed in a copper-clad laminate lends itself particularlywell, when used as a fusible protective device, to minimize the requiredfusing fiz'dt values. Because the burnback length of wire 3 is veryshort, i.e. because the current-carrying length of wire 3 situatedbetween solder joints 4 and inside of bore or passage 1a is very short,a single break formed by fusion of the wire 3 shown `in FIG 1 generatesbut a relatively small arc voltage. If lthe circuit voltage isrelatively high, many units of the lkind shown in FIG. 1 must bearranged in seriesas shown in FIGS. 2 4 of the above referred-to UnitedStates Patent and described in connection therewith-to achieve therequired increase of the arc voltage.

A further improvement of the arc voltage may be obtained by filling anappropriate synthetic resin into bore or passage 1a, thus plugging thelatter. While this increases to some extent the arc voltage per point ofbreak, it has no significant bearing on the available burnbacklengthwhich is substantially equal to the distance between the aforementionedsolder joints 4.

The hottest point of fusible wire 3 is midway between solder joints 4.The portions of wire 3 which are situated outside of bore or passage 1aare shunted by copper layer 2 and, therefore, are virtually not carryingany current. Upon occurrence of an excessive current wire 3 melts at itshottest point midway between solder joints 4 and burns back towardsolder joints 4. If the circuit is interrupted before, or at the time,the terminals of the arc reach solder joints 4, the performance of thestructure of FIG. l is satisfactory. If the arcing persists after thearc terminals have reached solder joints 4, the arc terminals burn deepinto solder joints 4 and metal layers 2, resulting in evolution `ofmetal vapors adverse to successful interruption of the faulted circuit.

To achieve successful interruption of a faulted circuit increase of thenumber of breaks is not always a good answer to generation ofinsutlicient arc voltages. Increasing of the number of breaks increasesthe initial arc Voltage. Continued arc elongation increases the arcvoltage in the more advanced stages of the interrupting process, ratherthan initially at the time of kindling of one or several arcs. Thereforeincreasing of the number of breaks and increasing of the length ofbackburn are not equivalents by any means.

The structure of FIGS. 2-4 is substantially the same as that shown inFIG. l, but includes additional means allowin-g a larger degree of arcelongation than the structure of FIG. 1 and being more readilyapplicable and less expensive to apply than the synthetic resin fillerof United States Patent 2,895,031.

In FIGS. 2-4 the same reference characters as in FIG. 1 With a primesign added have been applied to indicate like parts. Thus S is a stripof copper-clad organic insulating material comprising the innerinsulating layer ll' sandwiched between outer copper layers 2'. Thelatter are provided with transverse grooves 2a severing layers 2'. Wire3 preferably of silver is threaded through bore or passage 1a andconductively connected at 4 by solder joints to copper layers 2'. Sleeve5' of a heat-shrinkable organic insulating material surroundscopper-clad strip or support S'. Sleeve 5' tightly seals both ends ofbore or passage 1a' and ts tightly the portions of copper layers 2'adjacent passage 1a. Sleeve 5' is preferably made of a modifiedpolyoleiin whose molecules have been crosslinked by high-energy electronbeam radiation. Thus a compound is formed which does not melt or ow atany normal operating temperature to which sleeve is subjected.

As shown in FIG. 5 sleeve 5' comprises an inner layer 5a' of irradiationcross-linked polyoleiin tightly fitting strip S', an intermediate layer5b' of glass-cloth and an outer layer 5a' of irradiation cross-linkedpolyoleiin laminated together to form an integral sleeve structure. Toachieve best performance sleeve 5 ought to be a laminated structure asindicated above, rather than to consist only of a heat-shrinkablesubstance such as irradiation cross-linked polyolein. The intermediateglass cloth layer 5b' operates as an effective heat absorber afterfusion of wire 3' and kindling of an arc at the point of wire 3', and itincreases, in addition thereto, the mechanical strength of the compositesleeve 5'.

When an arc is kindled by fusion of wire 3' midway between solder joints4', the structure performs initially in the same way as the structure ofFIG. l, i.e. the heat of the arc evolves jets of relatively cool gasfrom the organic insulating layer 1'. These jets of relatively cool gaspenetrate, or diffuse, into the arc path and tend to accelerate thedielectric recovery thereof.

Assuming the rate of dielectric recovery to be insuicient, i.e. assumingthat the length of wire 3' between solder joints 4' is completelyconsumed or vaporized, and that the arc terminals therefor arrive atsolder joints 4' and at the portion of sleeve 4 immediately adjacentthereto. This results in generation of high gas pressures inside ofpassage 3 caused by the evolution of gas from sleeve 5' tending todrive, or propel, the arc terminals along copper layers 2 in oppositedirections longitudinally of strip S', This results in a significantelongation of the arc and a significant increase of the arc voltage `ata period of time following arc initiation rather than at the time of arcinitiation.

It will be understood that the arc which is kindled upon fusion of wire3' midway between points 4 is subjected to various actions. Therelatively cool gases evolved from the lateral walls of bore or passage1a mix turbulently with the hot gases at the immediate arcing zone andtend to decrease the ionization of the latter. There is anelectromagnetic or electrodynamic loop action tending to project theportion of the arc immediately adjacent copper layers 2 against theinner surface of sleeve 5'. The sleeve 5' reacts to the movement of thearc toward it by ejecting jets of relatively cool gas toward theportions of the arc moving toward it. The pressure inside bore orpassage 1a' tends to drive the arc terminals axially outwardly away frombore or passage 1a. When the arc terminals have moved to points 4 wherea conductive connection is established between copper layers 2 Iand wire3', the arc terminals are virtually in physical engagement with sleeve5', resulting in intense evolution of gases therefrom right into theterminals of the arc then. The terminals of the are move, or arepropelled, under the action of these various effects axially outwardlyas indicated by arrows R in FIG. 4. The terminals of the arc may causesome burning of copper layers 2', but the degree of burning is not sosevere .21S .'LO i 1 11 I.I.1,Oltlizte the arc terminals by formation ofdeep electron-emitting craters. Thus copper layers 2' form a kind of arcrunner, i.e. a means along which thev arc terminals are moved orpropelled and kept in motion. This is clearly apparent from thephotomacrograph of FIG. 6. FIG. 6 shows the arc path along one of layers2. The yarc path is a severe burning of layers 2' resulting from a veryonerous test close to the limit of the interrupting capacity of thedevice. Consequently the arc path is conspicuous and comprises numerousbeads of melted metal. Yet, nevertheless, as is clearly apparent fromFIG. 6, the arc terminals have been kept in motion -axially outwardlyresulting in progressive elongation of the arc path and in progressiveaddition of points of sleeve 5' as cross-gas-blast-generating means. Thearc path is immobilized except that the arc terminals are allowed totravel and, therefore, the arc path cannot move away from theimmediately adjacent gas-evolving increments of sleeve 5 and thus avoidthe intense de-ionizing action of the latter.

The structure of FIGS. 2-5 can readily be designed in such a fashion asto avoid a motion of the terminals of the arc beyond sleeve 5', i.e.outside of the area covered by sleeve 5'. At relatively severeinterruptions hot arc products will be ejected from sleeve 5', asindicated by the arrows P in FIG. 4. Hence it is necessary, ordesirable, to enclose the structure of FIGS. 2-5 in a tubular casingsubstantially in the same fashion as illustrated in FIG. 7 of the`aforementioned United State Patent 2,895,031 to Frederick J. Kozacka.

While FIGS. 2-5 show an interrupting device embodying this inventionincluding a current path having but one single break-forming point ofreduced cross-section formed by a length of wire, a plurality of suchpoints may be arranged in series as shown in FIGS. 3 and 4 of theaforementioned United States Patent 2,856,488 to Frederick I. Kozacka.

Structures of the kind shown in FIGS. 2-5, inclusive, are made bymounting a sleeve of a heat-shrinkable material loosely upon a wiredcopper-clad strip -as shown in FIG. 1 and thereafter heating the sleeveto cause shrinking thereof. The sleeve then shrinks under stress aroundthe wire support S and its metal layers 2. When the shrinking process iscompleted the heat-shrinkable sleeve is tightly clamped against bothends of the transverse wire-receiving passage 1a in strip or support S,and also tightly clamped against the portions of the metal overlay 2adjacent said passage.

It will be understood that though but Ione embodiment of the inventionhas been described in detail, the invention is not limited thereto. Itwill also be understood that the structure illustrated may be modifiedwithout departing from the spirit and scope of the invention as setforth in the accompanying claims:

I claim as my invention:

1. An electric fuse comprising in combination:

(a) a support of organic insulating material dening a transversepassage;

(b) metallic means cladding opposite sides of said support;

(c) a fusible element threaded through said passage having ends situatedoutside said passage and conductively connected to said met-allie means;and

(d) gas-evolving means responsive to fusion of said fusible element andkindling of an arc for establishing a gas-blast moving the terminals ofsaid arc beyond the break formed upon fusion of said fusible elementbetween said metallic cladding means along said metallic cladding meansin substantially opposite directions, said gas-evolving means includinga sleeve of a heat-shrinkable organic insulating material shrunk understress around said support and around .said metallic means, said sleevebeing tightly clamped against both ends of said passage and beingtightly clamped against the portions of said metallic means adjacentsaid passage.

2. An electric fuse comprising in combination:

(a) a support of organic insulating material defining a transversepassage;

(b) copper layers cladding opposite sides of said support;

(c) a fusible wire threaded through said passage having ends situatedoutside said passage and conductively connected to said layer; and

(d) gas-evolving means responsive to fusion of said fusible wire andkindling of an arc for establishing a gas-blast moving the terminals ofsaid are beyond the ends of said wire in substantially oppositedirections along said copper layers, said gas-evolving means including asleeveof a laminate having outer layers of a heat shrinkable organicinsulating material and an inner layer of glass-cloth, said sleeve beingshrunk under stress around said support and around said copper layers,and said sleevel being tightly clamped against both ends of said passageand being tightly clamped against the portions of said copper layersadjacent said passage.

3. An electric fuse comprising in combination:

(a) a support of organic insulating material defining a transversepassage;

(b) copper layers cladding opposite sides of said support;

(c) a fusible silver wire threaded through said passage having endssituated outside said passage and conductively connected to said copperlayers; and

(d) gas-evolving means responsive to fusion of said fusible wire andkindling of an arc for establishing a gas-blast moving the terminals ofsaid arc beyond the ends of said wire in substantially oppositedirections along said copper layers7 said gas-evolving means including asleeve of a laminate having outer layers of radiation cross-linkedheat-shrinkable polyolen and an inner layer of glass-cloth sandwichedbetween said outer layers, said sleeve surrounding said support and saidcopper layers, and said sleeve tightly sealing both ends of said passageand tightly fitting the portions of said copper layers adjacent saidpassage.

4. An electric fuse comprising in combination:

(a) a metal clad strip of organic insulating material defining atransverse passage;

(b) a length of wire -threaded through said passage and having one endconductively connected to the upper metal-cladding and another endconductively connect ed to the lower metal-cladding of said strip; and

(c) gas-evolving means responsive to fusion of said length of wire andkindling of an arc for establishing a blast of gas moving the terminalsof said arc beyond -the ends of said wire in substantially oppositedirections along said upper metal-cladding and along said lowermetal-cladding of said strip, said gas-evolving means including a sleevesurrounding said strip at the region of said transverse passage thereof,said sleeve comprising layers of heat shrinkable organic insulatingmaterial sandwiching therebetween a layer of fiber glass material, saidsleeve being shrunk under stress around said strip and being tightlyclamped against both ends of said passage and being tightly clampedagainst said strip along the entire length thereof surrounded by saidsleeve.

5. An electric fuse comprising in combination:

(a) a metal clad strip of organic insulating material defining atransverse passage;

(b) a length of wire threaded through said passage and having one endconductively connected to the upper metal-cladding and the other endconductively connected to the lower metal-cladding of said strip; and(c) gas-evolving means responsive to fusion of said 5 length of wire andkindling of an arc for establishing a blast of gas moving the terminalsof said arc beyond the ends of said wire in substantially oppositedirections along said upper metal-cladding and along said lowermetal-cladding of said strip, said gasevolving means including a sleevesurrounding said strip at the region of said transverse passage, saidsleeve comprising layers of heat shrinkable polyolefin sandwichingtherebetween a layer of fiber glass material, said sleeve being shrunkunder stress around said strip and being tightly clamped 4against bothends of said passage and being tightly clamped against said strip alongthe entire length thereof surrounded by said sleeve.

6. An electric fuse comprising in combination:

(a) a copper-clad strip of organic insulating material defining atransverse passage;

(b) a length of wire threaded through said passage and having one endconductively connected to the upper copper-cladding layer and the otherend conductively connected to the lower copper-cladding layer of saidstrip; and

(c) gas-evolving means for propelling arc terminals formed on said uppercopper-cladding layer and on said lower copper-cladding layer uponfusion of said wire in opposite directions longitudinally of said strip,said propelling means including a sleeve of a heat shrinkable organicinsulating material shrunk under stress around said strip.

7. An electric fuse comprising in combination:

(a) a copper-clad strip of insulating material defining a transversepassage;

(b) a length of wire threaded through said passage and having one endconductively connected to the upper cladding layer and the other endconductively connected to the lower cladding layer of said strip; and

(c) gas-evolving means for propelling arc terminals formed on said uppercladding layer and on said lower cladding layer upon fusion of said wirein opposite directions longitudinally of said strip, said propellingmeans including a sleeve of a laminate having layers of radiationcross-linked heat-shrinkable polyolefin sandwiching therebetween a layerof glasscloth, said sleeve being shrunk under stress around said stripand being tightly clamped against both ends of said passage.

References Cited by the Examiner UNITED STATES PATENTS 3/1958 l/l9597/l959 5/1962 6/1962 Folta.

6/ 1963 Kirkpatrick et al.

Blatz. Kozacka. Danchuk.

Kozacka ZOO-131

1. AN ELECTRIC FUSE COMPRISING IN COMBINATION: (A) A SUPPORT OF ORGANICINSULATING MATERIAL DEFINING A TRANSVERSE PASSAGE; (B) METALLIC MEANSCLADDING OPPOSITE SIDES OF SAID SUPPORT; (C) A FUSIBLE ELEMENT THREADEDTHROUGH SAID PASSAGE HAVING ENDS SITUATED OUTSIDE SAID PASSAGE ANDCONDUCTIVELY CONNECTED TO SAID METALLIC MEANS; AND (D) GAS-EVOLVINGMEANS RESPONSIVE TO FUSION OF SAID FUSIBLE ELEMENT AND KINDLING OF ANARC FOR ESTABLISHING A GAS-BLAST MOVING THE TERMINALS OF SAID ARC BEYONDTHE BREAK FORMED UPON FUSION OF SAID FUSIBLE